This invention relates generally to mode suppression in microwave circuits and more particularly to a means for broadband mode suppression in integrated circuits.
Where electromagnetic wave energy is transmitted in hollow pipe like waveguides it is well documented that, dependent on the size and shape of the waveguide, characteristic electric and magnetic field configurations are generated. These fields are referred to as "modes". Modes are a function of frequency as well as physical characteristics of the waveguide.
Generally it is well established that the larger the wave guide and the higher the frequency the more numerous the modes created. While it is most desirable to confine these modes to a single fundamental mode, frequently devices added to the waveguide as mode suppressors actually induce high frequency moding not compensated for by fundamental mode suppression.
A more recent problem that has tended to effect waveguide transmission is increase in functions that are added to microwave integrated circuit modules. These devices known in the art as "super modules" have severe difficulties relating to moding and circuit isolation. Most microwave integrated circuits are fabricated in substrates with a low loss, high dielectric constant using microstrip circuit topology.
When utilizing a microstrip any circuit discontinuities vertical to the top plane of the microstrip circuit tend to both radiate and couple RF energy relative to each other. Although great care is exercised in the design of such circuits in an effort to minimize such discontinuities, chip capacitors, RF chokes, inter-line connects and other devices all contribute to the basic radiation problem.
As more circuit functions are added to the module, the size or surface area required to implement the circuit increases. This increase in turn requires the use of larger external metal boxes or enclosures which serve not only as a mechanized holder providing physical protection, but also, furnishes necessary RFI and EMI protection to both incoming and outgoing RF signals. These enclosures are frequently machined from aluminum.
There is a critical set of internal dimensions for the enclosure which depend upon frequency and when reached will support a waveguide or cavity mode. This RF energy is launched from the aforementioned circuit discontinuities. Although there are a substantial number of possible results of these highly undesired waveguide modes examples include, circuit oscillation, so-called resonant suck-outs within band, loss of isolation between circuits employing dual or multi channel designs and bandwidth shrinkage problems.
A well known method of suppressing RF moding within a metal enclosure is with the precise placement of metal posts connecting the top cover to the bottom ground surface. The post being parallel to the E field produces an RF short. When the post is placed in the center of the box, in the direction of RF propagation, then the minimum effect propagation frequency will be appropriately doubled in the region of the post. While this technique is very effective for mode suppression of the fundamental frequency, high frequency moding can exist and actually be induced by the addition of fundamental mode suppressors.
The proposed invention discloses a low cost technique for producing very broad band mode suppressors that are effective not only at the fundamental frequency but also at frequencies above and below the fundamental frequency.